How many 4 digit combination's can you get from numbers 1-20?

Answer 1

This isn't a very specific question ... by four digits, do you mean four numbers? Furthermore, are the digits or numbers you use allowed to repeat?

Let's assume you meant numbers, not digits. In this case, for example, you could be trying to create a locker combination, and the numbers you can use are 1 through 20, and the combination has four "blank" spaces.

1. If the numbers can repeat, then each spot has 20 possibilities, so the total number of combinations is 20 for the first spot, 20 for the second, 20 for the third, and 20 for the fourth. That is, 20*20*20*20 = 160,000 combinations.

2. If the numbers cannot repeat, then the first spot can be taken up by any of 20 numbers, the second spot only by 19 (because you can't repeat the first number, whatever it was), the third spot by 18, and the fourth spot by any one of the 17 remaining numbers. So that makes 20*19*18*17 possibilities = 116,280 combinations.

If this is not what you meant, and you wanted digits, remember that every number above and including 10 has two digits, so you'll want to factor that into your calculations.